Quality Control of Ionospheric Corrections ION ITM 2015 Dana Point, CA Jan. 26-28, 2015 Quality Control of Ionospheric Corrections for Adoption of Wide Area Augmentation at the Low Latitude Regions T. Sakai, T. Aso, K. Hoshinoo, and K. Ito Electronic Navigation Research Institute, Japan

Introduction QZSS (Quasi-Zenith Satellite System) program: ION ITM Jan. 2015 Introduction QZSS (Quasi-Zenith Satellite System) program: Regional navigation service broadcast from high-elevation angle by a combination of three or more satellites on the inclined geosynchronous (quasi-zenith) orbit. Broadcast GPS-like supplemental signals on three frequencies and two augmentation signals, L1-SAIF and LEX. L1-SAIF (Submeter-class Augmentation with Integrity Function) signal offers: Submeter accuracy wide-area differential correction service; Integrity function for safety of mobile users; and Ranging function for improving position availability; all on L1 single frequency. ENRI has been developing L1-SAIF signal and experimental facility: L1-SAIF signal achieves good accuracy less than 1 meter in RMS manner at the mainland of Japan. Ionosphere disturbance sometimes degrades the position accuracy, especially at the low latitude regions including the southwestern Islands of Japanese territory. In order to improve the accuracy at the low latitude regions during ionospheric storm, quality control of ionospheric corrections may be introduced.

QZSS Concept Broadcast signal from high elevation angle. ION ITM Jan. 2015 QZSS Concept QZS GPS/GEO Broadcast signal from high elevation angle. Applicable to navigation services for mountain area and urban canyon. Augmentation signal from the zenith could help users to acquire other GPS satellites at any time. Footprint of QZSS orbit. Centered at 135E. Eccentricity 0.075, Inclination 43deg.

Space Segment: QZS-1 L1-SAIF Antenna 25.3m ION ITM Jan. 2015 Space Segment: QZS-1 L-band Helical Array Antenna L1-SAIF Antenna Laser Reflector C-band TTC Antenna Radiation Cooled TWT TWSTFT Antenna 25.3m Successfully launched on Sept. 11, 2010 and settled on Quasi-Zenith Orbit (IGSO). Nickname: “Michibiki” Mass 4,020kg (wet) 1,802kg (dry) (NAV Payload：320kg) Power Approx. 5.3 kW (EOL) (NAV Payload: Approx. 1.9kW) Design Life 10 years

Broadcast Ranging Signals ION ITM Jan. 2015 Broadcast Ranging Signals Supplemental Signals Supplemental Signals by JAXA GPS-like L1C/A, L2C, L5, and L1C signals (PRN 193) working with GPS. Improves availability of navigation. Minimum modifications from GPS signals. QZS-1 satellite L1-SAIF by ENRI SBAS-like C/A code (PRN 183) signal on GPS L1 freq.; Reasonable performance for mobile users. Augmentation to GPS; Potentially plus GLONASS and Galileo. LEX by JAXA For carrier-based experimental purposes. Original CPSK signal on E6 frequency. Member organizations may use LEX as 2kbps experimental data channel. Augmentation Signals

SAIF： Submeter-class Augmentation with Integrity Function ION ITM Jan. 2015 L1-SAIF Signal Ranging Function Error Correction Integrity Information GPS Constellation Ranging Signal L1-SAIF Signal QZS satellites SBAS-like augmentation signal on PRN 183. Three functions by a single signal: ranging, error correction (Target accuracy: 1m), and integrity. User receivers can receive both GPS and L1-SAIF signals with a single antenna and RF front-end. See IS-QZSS for detail (Available at JAXA HP). User GPS Receivers SAIF： Submeter-class Augmentation with Integrity Function

ENRI L1-SAIF Master Station ION ITM Jan. 2015 ENRI L1-SAIF Master Station L1-SAIF Master Station (L1SMS): Generates L1-SAIF message stream in realtime and transmits it to QZSS MCS developed by and installed at JAXA. Installed at ENRI, Tokyo; 90km from JAXA Tsukuba Space Center. Dual frequency GPS measurements at some locations in Japan (monitor stations) necessary to generate L1-SAIF messages are sent from GEONET in realtime. L1SMS GEONET QZS QZSS MCS GNSS Satellites Measure- ments L1-SAIF Message GSI Server (Tokyo) ENRI JAXA TKSC (Tsukuba) L1-SAIF Signal Ranging Signal K-band Uplink

L1-SAIF Performance GPS Only Result Horizontal Error Vertical 1.45 m ION ITM Jan. 2015 L1-SAIF Performance Standalone GPS L1-SAIF Augmentation GPS Only Result 6 reference stations User location for this test L1-SAIF expe- rimental area Horizontal Error Vertical 1.45 m 2.92 m 6.02 m 8.45 m System Standalone GPS 0.29 m 0.39 m 1.56 m 2.57 m w/ L1-SAIF RMS Max Example of user position error at Site 940058 (Takayama). Realtime operation with MSAS-like 6 reference stations in Japan. Period: 19-23 Jan. 2008 (5 days). Note: Results shown here were obtained with geodetic-grade antenna and receivers at open sky condition.

Problem: Ionosphere Ionosphere Density (NASA/JPL) ION ITM Jan. 2015 Problem: Ionosphere Ionosphere Density (NASA/JPL) The largest error source: Ionospheric propagation delay. Varies on the local time, solar activity, earth magnetic field, and so on. Cannot be predicted; Causes large effect in the low magnetic latitude region.

Accuracy at Southwestern Island ION ITM Jan. 2015 Accuracy at Southwestern Island LT 14:00 At Southwestern Island (960735 Wadomari) At Northernmost City (950114 Kitami) During severe ionospheric storm condition (Kp~7+), position accuracy with differential correction largely degrades at the Southwestern Islands. The effect is not so large at the mainland of Japan. It is known that increase of the number of GMS shows a little improvement.

Actual Ionosphere Corrections ION ITM Jan. 2015 Actual Ionosphere Corrections PRN20 PRN28 5m At Southwestern Island At Northernmost City Ionospheric correction continuously differs from the true delay by 5m or more. Degradation of position accuracy during storm is due to inaccurate ionospheric correction.

L1-SAIF Ionospheric Correction ION ITM Jan. 2015 L1-SAIF Ionospheric Correction 120 150 180 30 60 Longitude, E Latitude, N 15 45 IGP Vertical ionospheric delay information at IGPs (ionospheric grid point) located with 5-degree interval will be broadcast to users. User receiver computes vertical ionospheric delays at IPPs with bilinear interpolation of delays at the surrounding IGPs. Vertical delay is converted to slant delay by multiplying so-called obliquity factor which is a function of elevation angle. IGP IPP

Message Type 26: Ionospheric Delay Information ION ITM Jan. 2015 Broadcast Message MT26: Broadcast Ionospheric Vertical Delay Contains vertical delay and its uncertainty at IGP. A MT26 message contains information for 15 IGPs. Each IGP has two attributes: vertical delay and its uncertainty (GIVE). Message Type 26: Ionospheric Delay Information Repeat Content Bits Range Resolution 1 IGP Band ID 4 0 to 10 IGP Block ID 0 to 13 15 IGP Vertical Delay 9 0 to 63.875 m 0.125 m GIVEI 0.3 to 45 m or Not Monitored (Table) IODI 2 0 to 3 Spare 7 —

Planar Fit and GIVE GIVE Equation ION ITM Jan. 2015 Planar Fit and GIVE Cutoff Radius Vertical Delay Fit Plane IPP IGP Planar Fit: An algorithm to estimate vertical delay at an IGP. Developed for WAAS; MSAS and L1-SAIF employ the same algorithm. Assume ionospheric vertical delay can be modeled as a plane. Model parameters are estimated by the least square fit. GIVE (grid ionosphere vertical error): Uncertainty of the estimation basically computed from residuals of fit and including spatial and temporal threats. GIVE Equation Formal Sigma Spatial Threat Temporal Threat

ION ITM Jan. 2015 Estimation Error Difference between observation and estimation at user IPPs Difference, m 4:30 At Northernmost City (950114 Kitami) Over 2.5m Difference, m 4:30 At Southwestern Island (960735 Wadomari)

Quality of Estimation The MCS Can Examine the Quality of Estimation. ION ITM Jan. 2015 Quality of Estimation The MCS Can Examine the Quality of Estimation. Based on the observations used for estimation. Ex. Storm Detector Algorithm: Chi-Square test for residual errors (already implemented). Use the difference between observation and estimation for all IPPs: Ionospheric delay observed at an IPP observed by MCS; and The associate delay estimation at the location of the IPP interpolated based on surrounding four IGP delay information generated by the MCS. The large difference means invalidity of ionosphere estimation used for correction at the location of IPP. Difference, m | I(fIPP_i, lIPP_i) - IIPP_i | ^ Difference between observation and estimation at the IPPs of MCS

Quality Control of Corrections ION ITM Jan. 2015 Quality Control of Corrections Action to Detected Invalidity. Due to the equatorial anomaly, irregularity of ionosphere may exist in south side. In case that the difference at an IPP is larger than the threshold, all IGPs in the south from the IPP are set to: ‘Not Monitored’ (NM) with threshold THNM (QC-NM); or Maximum GIVE (45m) with threshold TH45 (QC-45). Difference, m QC-45: IGPs in the south from 20N are set to 45m due to two IPPs (red circled) having the difference larger than TH45 = 3.0m. QC-NM: IGPs in the south from 15N are set to NM due to an IPP (black circled) having the difference larger than THNM = 5.0m.

Quality Control of Corrections ION ITM Jan. 2015 Quality Control of Corrections For All IPPs For All IGPs For All IGPs > fIGP_k : fIPP_i Compute Estimation for IPP i: I(fIPP_i, lIPP_i) ^ Planar Fit Algorithm for IGP k < |I(fIPP_i, lIPP_i)-IIPP_i| : TH ^ Set GIVE = NM or 45m for IGP k > < Observation Estimation Conventional Planar Fit Implementation New: Quality Control Algorithm

Experiment Setup Observation Data from GEONET: ION ITM Jan. 2015 Experiment Setup Observation Data from GEONET: GPS network operated by Geospatial Information Authority of Japan. Archive of 30-sec interval data. L1-SAIF Master Station: Runs in offline mode with RINEX files. Using 6 GEONET stations (a) to (f) near MSAS/L1-SAIF GMS (Ground Monitor Station) locations to generate augmentation message. User Stations: 5 stations from North to South: (1) to (5) for performance evaluation. Compute position estimates with the L1-SAIF message output from L1-SAIF Master Station

Baseline Performance At Southwestern Island (5) Wadomari ION ITM Jan. 2015 Baseline Performance At Southwestern Island (5) Wadomari At Northernmost City (1) Kitami Period: 2011/10/23 to 2011/10/26 (severe storm: Kp~7+) L1SMS Parameters: # of GMS = 6, Iono Correction = Planar Fit

Quality Control (QC-45) At Southwestern Island (5) Wadomari ION ITM Jan. 2015 Quality Control (QC-45) At Southwestern Island (5) Wadomari At Northernmost City (1) Kitami Quality Control with TH45=1.0m (QC-45) and THNM= (No QC-NM). Possibility of some improvement at southwestern islands. Availability of position solution is not affected.

Quality Control (QC-45 and QC-NM) ION ITM Jan. 2015 Quality Control (QC-45 and QC-NM) At Southwestern Island (5) Wadomari At Northernmost City (1) Kitami Quality Control with TH45=2.0m (QC-45) and THNM=3.0m (QC-NM). Improvement is not clear. Availability of position solution is degraded.

Effects of Quality Control ION ITM Jan. 2015 Effects of Quality Control 10% No loss of Availability Apply QC-45 Only Apply QC-45 and QC-NM QC-45 (Quality Control with setting Max GIVE) has a potential of 10% improvement. QC-NM (Quality Control with setting Not Monitored) has unclear improvement and degraded availability.

Conclusion ENRI has been developing L1-SAIF signal: ION ITM Jan. 2015 Conclusion ENRI has been developing L1-SAIF signal: Signal design: GPS/SBAS-like L1 C/A code (PRN 183). Planned as an augmentation to mobile users. Ionosphere disturbance is a concern: L1-SAIF signal achieves good accuracy less than 1 meter in RMS manner at the mainland of Japan. Ionosphere disturbance sometimes degrades the position accuracy, especially at the Southwestern Islands of Japanese territory. This time we tried Quality Control of ionospheric corrections based on the difference between observation and estimation at IPPs. Expected performance improvement: ~10% in RMS. Further Investigations will include: Other methods to improve the quality of ionospheric corrections for wide area augmentation. Consideration of using external data sources (space weather and other sensors). Performance improvement at other Asian Countries.